In the automatic testing of electrical circuits, test probes of various configurations are used, depending upon such factors as the type of electrical device under test, the spacing between test points, and the like. The present invention is applicable, in part, to the testing of light-emitting circuit components that are present on printed circuit boards. These circuit components can include light-emitting diodes (LED's), optical displays, and opto-isolators, for example. These devices require optical testing of their light-emitting functions to determine whether light emission exceeds a minimum required intensity, whether light intensities produced by a number of components are uniform, and the like.
U.S. Pat. No. 4,808,815 to Langley discloses a test fixture used in conjunction with conventional automatic test equipment to perform optical testing of a wide variety of light-emitting devices. The fixture comprises a plurality of individual optical probes arranged in a preselected pattern. Each probe is disposed in close proximity to a light-emitting component. A fiber optic cable connects each probe to a detector. Each of the detectors produces an electrical output signal related to the intensity of the light conducted by its fiber optic cable. The output signals of the detectors are electrically connected to a conventional automatic test system for converting light intensity readings into electrical signals for testing the light-emitting functions of the circuit components of a unit under test.
In addition to directly testing the light-emitting functions of printed circuit board components, such as LED's, testing the electrical circuit continuity of electrical circuits or circuit components on a printed circuit board also can be performed optically. For instance, digital electrical signals can be converted at the board level to corresponding optical signals which can then be detected by optical fiber test probes for use in functional testing of the circuits or circuit components on the board. This avoids use of conventional electrical test probes. At very high digital signal pulse frequencies, electrical test probes can require shielding from surrounding electromagnetic fields, which adds to the expense of the test probes and increases the center-to-center mounting distance. This problem is avoided by using optic fiber test probes which produce a clean signal without being affected by surrounding electromagnetic fields. Optical fibers also have no line loss when compared with electrical signals which can degrade with noise.
There are variety of problems that must be addressed in developing an optical fiber test probe for use in automatic test equipment. The optical fiber probes must meet certain space requirements so they can easily match the close spacing pattern between LED's or other optical circuit elements that may be present on a circuit board.
Each fiber optic probe assembly contains a number of components, and the assembly should be arranged so that it requires relatively few components that can be quickly and easily assembled by mass production assembly techniques.
A fiber optic test probe commonly requires making direct contact with the light-emitting source during testing. This typically requires that the probe assembly include some sort of compression spring so that contact during test is spring-biased, and the spring then acts as a return spring when the test force is released. The fiber optic test probe assembly should be arranged so that spring forces applied during repetitive test sequences do not adversely affect reliable long-term operation of the fiber optic test probe. Moreover, the optical fibers used in these test probes can be of miniature size, say 0.005 inch in diameter, and the repetitive functioning of the probe during testing should not apply undue loading to the optical fiber.
The fiber optic probe assembly also should be designed so that, if replacement is necessary, the probe can be removed and replaced reliably, with minimal downtime and expense.